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Old January 2nd, 2004, 10:50 AM   #1
naserals
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Load Cells - Simatic S7-400

Port silos plant is working fine except for tracking loaded materials. We decided to install load cells under the load bins. Now, I have some questions to tighten my Project Plan:

1. Is the vendor free of installing any load cell brand?
2. The vendor is going to group each load bin's load cells in one junction box which is abvious. Then connect the jbox output to the PROFIBUS and then to the PROFIBUS-DP module. How many mv needed by the PROFIBUS-DP?
3. Do we need to update the PLC configuration? What do we have to do?

Please notice that I'm SIMATIC S7 begginer. So detailed answers are required.
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Old January 2nd, 2004, 03:53 PM   #2
Pierre
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Re: Load Cells - Simatic S7-400

Quote:
Originally posted by naserals

1. Is the vendor free of installing any load cell brand?
Yes! They are only variable resistors. You must install a set identical ones under a bin.

Quote:
Originally posted by naserals

2. The vendor is going to group each load bin's load cells in one junction box which is abvious. Then connect the jbox output to the PROFIBUS and then to the PROFIBUS-DP module. How many mv needed by the PROFIBUS-DP?
The junction box serves to balance the load cells resistances, you just cannot only connect them in parallel.
You profibus x-mitter is independant of your load cells working schemes. You must provide a suitable power supply to those load cells. The x-mitter will only transmit the resulting voltage. Check the specs of the load cells in mV/V and figure out the x-mitter range according to the supply you have provided.
Quote:
Originally posted by naserals

3. Do we need to update the PLC configuration? What do we have to do?
You must of course use blocks to communicate the load cells node values.
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Old January 2nd, 2004, 04:10 PM   #3
naserals
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Thanks Pierre.

Would you please go in detail in the second question? I mean would you give me an example.

Regarding the third question, do I have to add data blocks to fetch Inputs. I know it's an annoying questions but how would I do that. Can I do it myself or should I assign this part to an expert?
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Old January 2nd, 2004, 07:27 PM   #4
Pierre
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When you use a load cell, you will "see" in the specification that its "sensitivity" is XmV/V.

This means that for a load cell of max. capacity of 100 Kg and a signal of 2.5 mV/V at 10Vdc...

You will have an output from this load cell of 25.00 mV when you have a weight of 100Kg on it AND an exitation voltage of 10 Vdc.

You can apply a Voltage of 5 Vdc but then you would only have half the output...

Soooo! Now you have a signal from a load cell. Since you could have more than one load cell on any given system being weighted, you must ADD these signals.

You cannot wire them in serie because each would not get the same exitation voltage, you cannot wire them in parallel because them signals would be interfearring one another... Whats to do?

You need a junction to do this addition. Its the load cell junction box. They are call "Load Cell Summing junction Boxes".

They are circuits doing this addition and outputing only the results.

So you would still get a signal corresponding to the real weight. They have a procedure for calibration and some even have digital calibration and fault detection features.

So now you have a signal from 0 to XX mV. You need to get this in your remote PLC through a Profibus Network.

You do has any other devices. Set an address to the device. Set a communication program and retreive the Data. Then play with it.

As to the program, you will find it very easy ONCE ITS BEEN DONE. So my best advice is to get an expert to do it "not for you but with you". He should show you how to write the addresses, to write the data gathering blocks and to move thos results into a meaningfull array of registers. After this you should be able to do it on your own the next time. This way you will save a lot of time and avoid mistakes. Hopfully.
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Last edited by Pierre; January 2nd, 2004 at 07:31 PM.
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Old January 3rd, 2004, 04:42 AM   #5
seppoalanen
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Quote:
1. Is the vendor free of installing any load cell brand?
There are many kind of loadcells in the marked. Some works as resistance, some works as inductance...
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Old January 3rd, 2004, 10:56 AM   #6
naserals
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Thank you very much Pierre. I can catch the road with these details.

I just would like to breif you about my understanding of the whole setup:

1. Load Cells/JBoxes installation and cabling up to the PROFIBUS-DP module.
2. Each JBox will be connected to a PROFIBUS-DP module.
3. JBoxes will be giving a signal between 0 - X mV.
4. Using Step7 (SIMATIC Manager), we can configure the SIMATIC S7-400 to read JBoxes inputs.
5. We add these inputs as DP Slaves in the PLC hardware configuration.
6. We assign them unique I Addresses (No idea how many bytes).
7. We create a Variable Table to include above Input addresses.
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Old January 3rd, 2004, 06:57 PM   #7
dchartier
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Hello, Naserals;
Your description appears very adequate, except for point #3:
the Profibus module will not give a mV signal; generally it will send to the Profibus master a pre-scaled value, on a range of 0-27648 for a Siemens analog value).
Open the *.gsd file that comes with the Profibus slave module (you will need it to set it up in the Step 7 environment). Use any text editor to look at it; you can probably deduce from the different sections the number of words it will write and the range it will scale the signals to.
Hope this helps,
Daniel Chartier
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